1. Field of the Invention
The present invention relates to an optical switch and a micro-electro-mechanical system (MEMS) package that switch routes of beams in optical communication.
2. Description of the Related Art
Nowadays, to handle the explosion of Internet traffic, networks are rapidly being changed to optical networks based on wavelength division multiplexing (WDM) communication. Although the WDM communication at present mostly takes a form of a point-to-point network, such communication is expected to be advanced to a ring network or a mesh network in the near future.
Moreover, it is expected that add/drop of any given wavelength, optical cross connect (OXC) without conversion to electrical signals, and the like will be enabled at each node forming the network, and that dynamic path setting/releasing based on wavelength information will be performed.
In this way, photonic network technology that makes the most of optical technology is being developed (refer to, for example, “Institute of Electronics, Information and Communication Engineers Journal”, Feb. 1, 2002, February 2002 issue, pp. 94-103). A wavelength selective switch related to the present invention is an optical switch that is arranged in a node of a ring network or a mesh network, and has a function of selecting an output port to which a signal is to be sent depending on a wavelength thereof.
FIG. 14 is a plan view of a conventional wavelength selective switch. As shown in FIG. 14, a conventional wavelength selective switch 140 includes a port group 141, a collimating lens group 142, a dispersing element 143, a converging lens 144, a λ/4 plate 145, and a MEMS package 146.
A WDM beam having plural wavelengths input through the port group 141 is collimated by the collimating lens 142 to be transmitted to the dispersing element 143. Generally, a diffraction grating or the like is used as the dispersing element 143. The diffraction grating is an optical device that is formed with a glass substrate on which a number of parallel grooves are arranged at uniform intervals. The plural wavelength components of the WDM beam are input into the diffraction grating at a specific angle and the diffraction grating outputs the wavelength components at different output angles, according to wavelength, utilizing the diffraction phenomenon of light.
The dispersing element 143 disperses the WDM beam into components according to wavelength. The dispersed components are transmitted to the converging lens 144. Respective beams of the components that have passed through the converging lens 144 are transmitted to the MEMS package 146 via the λ/4 plate 145. The MEMS package 146 includes plural MEMS mirrors 147 arranged in an array. The MEMS mirrors 147 are movable reflectors that independently reflect the beams respectively converged by the converging lens 144.
The beams reflected by the MEMS package 146 pass through the λ/4 plate 145, the converging lens 144, the dispersing element 143, and the collimating lens group 142 again, and then are output from the port group 141. Hereinafter, the spectral direction, with respect to the beams dispersed by the dispersing element 143 according to wavelength, is referred to as a wavelength distribution direction. Furthermore, a direction of a beam that is collimated by the collimating lens group 142 and transmitted to the dispersing element 143 is referred to as an optical axis direction.
FIG. 15 is a front view of the conventional wavelength selective switch. As shown in FIG. 15, the port group 141 includes ports 0 to 4 that are arranged in a direction different from the wavelength distribution direction. The MEMS mirror 147 in the MEMS package 146 is freely rotatable about an axis oriented in the wavelength distribution direction, and reflects beams emitted from the converging lens 144 through the λ/4 plate 145 at a variable tilt angle. The wavelength selective switch 140 switches ports through which a beam is input and output by changing the tilt angle of the MEMS mirror 147.
Hereinafter, the direction in which the ports 0 to 4 are arranged is referred to as a port arranging direction. In the MEMS package 146, the MEMS mirror 147 is airtightly sealed in a casing to avoid the effects of humidity or a foreign substance. With consideration of mechanical strength and light transmission, generally, sapphire glass is used for a transmissive window 148 provided in the casing of the MEMS package 146 (for example, Japanese Patent No. 3777045).
Sapphire glass is a uniaxial crystal, and is birefringent in some crystal axial directions. To eliminate the effect of birefringence of sapphire glass, a structure in which the direction of C-axis of sapphire glass forming the transmissive window 148 and the direction of beams that pass through the transmissive window 148 are identical has been disclosed in, for example, Japanese Patent Laid-Open Publication Nos. H8-148594 and 2005-136119.
The wavelength selective switch 140 includes more than one element that causes polarization dependent loss (PDL) as typified by a dispersing element such as a diffraction grating. Therefore, it is difficult to keep PDL in the entire wavelength selective switch 140 lower than a specified value for the system, just by suppressing PDL in each individual element. For this reason, the λ/4 plate 145 is arranged between the MEMS package 146 and the converging lens 144 to cancel PDL.
The λ/4 plate 145 obtains a λ/4 phase difference between ordinary and extraordinary light of a beam that passes through the λ/4 plate 145. Therefore, the polarization of a beam from the port group 141 to the MEMS package 146 and that of a beam reflected from the MEMS package 146 toward the port group 141 become orthogonal with respect to each other. As a result, PDL is canceled. Thus, PDL of beams that are output from the wavelength selective switch 140 is reduced.
In the above conventional technique, the λ/4 plate 145 is provided in the wavelength selective switch 140. Therefore, the number of optical elements constituting the wavelength selective switch 140 increases, and a holding mechanism to adjust and fix the λ/4 plate 145 on an optical path is required, thereby increasing the size of the wavelength selective switch 140 and complicating manufacturing of the wavelength selective switch 140.
In addition, since the λ/4 plate 145 is expensive, the cost of the wavelength selective switch 140 becomes high. Moreover, as the λ/4 plate 145 is fragile, if the λ/4 plate 145 is provided in the wavelength selective switch 140, handling of the wavelength selective switch 140 becomes difficult. For example, in the case of the λ/4 plate 145 made of quartz, when the wavelength to be used is 1550 millimeters (mm), thickness of the λ/4 plate 145 at zero-order is to be 50 micrometers (μm), making the λ/4 plate 145 fragile.